In a stretch blowing machine as known from EP 1 066 149 A, each stretching process is controlled by means of an electric servomotor, e.g. a linear motor, via the electronic programmable control unit. In doing so, a process is conducted in which the force the servomotor has transmitted to the stretching rod is variable, e.g. in order to avoid damages of the bottom of the preform, namely in a control loop, the force automatically resulting from the motor current and a sensor or force meter position. The control unit controls predetermined force curves. Although the force the servomotor transmitted to the stretching rod is varied, it is continuously being increased until the stretching process has been terminated so that the force will have reached a maximum value by then.
In a stretch blowing machine as known from WO 2008/098565 A, a rigid stretching system is provided at the stretching rod, an electric servomotor, e.g. a linear motor and a mechanical coupling. The control unit comprises a controller for the force-controlled stretching of the preforms. The force the servomotor transmitted to the stretching rod is measured by detecting the motor current and is varied over the stretching distance. The stretching rod position is detected by sensors provided at the motor, respectively. Predetermined force/distance or force/time profiles are being executed by means of said control unit.
As soon as the stretching process has been terminated, i.e. as soon as the preform has been stretched to full length, the lower end of the stretching rod can press into the bottom of the preform so as to avoid that the bottom drifts away to the side (off-center protection) which might otherwise result in an uneven thickness of the material over the circumference of the container. Such so-called over-pressing can be easily controlled in pneumatic stretching bar drive systems, as the pneumatic cylinder operates in an abutting manner and a pneumatic drive system is not rigid. In contrast thereto, stretching bar drive systems having an electric servomotor are rigid systems, with the imminent risk for each stretching operating that the servomotor tries to move towards a desired position at maximum force, yet without being able to reach same so that thermal overload will occur.